Process for enhancing magnetic properties of metal powder by heat treating with salt

ABSTRACT

A process for enhancing the magnetic properties of small particulate ferromagnetic powders, e.g. particles below the critical diameter for which an untreated particle&#39;&#39;s domainboundary energy equals its magnetostatic energy, the process comprising heat treating said particles. In the more favorable embodiments of the invention, the particles are heated in intimate association with a particulate refractory material which serves as a shield for effectively preventing excessive sintering of the metal particles.

United States Patent n 1 Ehrreich et a1.

1 1 PROCESS FOR ENHANCING MAGNETIC PROPERTIES OF METAL POWDER BY HEATTREATING WITH SALT [75] lnventors: John E. Ehrreich, Wayland; Adrian R.Reti, Cambridge, both of Mass.

[73] Assignee: Graham Magnetics, Inc., Graham,

Tex.

[22] Filed: Mar. 24, 1971 [21] Appl. No.: 127,851

[52] US. Cl. 148/105, 75/O.5 AA, 75/0.5 BA,

148/102, 148/103, 148/112 [51] Int. Cl. H01f 1/02 [58] Field of Search75/0.5 AA, 0.5 BA;

[5 6] References Cited UNITED STATES PATENTS 2,503,947 4/1950 Haskew264/111 2,885,366 5/1959 ller. 106/308 3,663,318 5/1972 Little et a1.148/105 3,669,643 6/1972 Bagley er a1. 148/105 3,672,867 6/1972 Little148/105 3,567,525 3/1971 Graham et a1... 148/31.57 3,255,052 6/1966Opitz 148/105 [4 1 Aug. 28, 1973 1,838,831 12/1931 Hochheim et al.148/105 2,974,104 3/1961 Paine et a1 148/105 X 3,188,247 6/1965 de Voset al 148/101 3,206,338 9/1965 Miller et al. 148/105 3,535,104 10/1970Little, Jr. et a1. 75/0.5

OTHER PUBLICATIONS Thrush, P. W. et a1., (Edit.) Dictionary of Mining,Minerology & Related Terms (US. Dept. Interior), 1968, pp. 908 & 956.

Primary Examiner-L. Dewayne Rutledge Assistant Examiner-W. R.Satterfield Atromey--Cesari & McKenna [57] ABSTRACT 7 Claims, NoDrawingsPROCESS FOR ENHANCING MAGNETIC PROPERTIES OF METAL POWDER BY HEATTREATING WITI-I SALT BACKGROUND OF THE INVENTION This invention relatesto a process for making magnetic particles formed of exceptionally smallmetal particles, and to the particles formed thereby.

In recent years there has been a great deal of activity in making smallmetallic particles for use in various fields of technology. For example,metal powders are utilized in catalytic-type processes wherein they maybe carried on an inert carrier or may be used directly. Metal powdersalso find some utility in pyrotechnic formulations and in manufacture ofspecial-purpose mixtures such as, for example, might be used inmanufacture of magnets. Metal powders also find use in manufacture ofmagnetic tapes such as those used in information retrieval systems.

Very small metallic particles are particularly useful in a number ofthese applications because of their high surface area or some other suchinherent property. It is often convenient that such very small metallicparti cles be magnetic. Magnetic characteristics not only make themuseful for many special applications, but also make them more easilyrecoverable and easier to manipulate. However, it has remained a problemto produce metallic particles of high magnetic character when theparticles are very small, e.g. below the socalled critical diameter of agiven metal.

As disclosed by Thomas O. Paine at pages 149150 of Magnetic Propertiesof Metals and Alloys published by American Society for Metals,Cleveland, 0. (1959), the critical diameter is that for which aparticles domain-boundary energy equals its magnetostatic energy.

Clearly, then, the more ideally-arranged magnetic particles will have ahigher magnetostatic energy than less ideally-arranged magneticparticles of the same size. This effect has been used by investigatorssuch as Miller and Oppegard who, in work described in U. S. Pat. No.3,206,338, caused very small particles to be produced in a magneticfield, thereby enhancing the arrangement of the atoms within eachparticle to provide a relatively ideal magnet; and, consequentlyobtaining greater magnetostatic energy for each particle than would havebeen achieved without use of a magnet. The resulting particles exhibitmagnetic behavior at unusually small particle sizes, i.e. particle sizesin the order of 0.01 micron (100 Angstroms) in crosssectional directionand 0.05 microns (500 Angstroms) in length.

This utilization of a magnetic field, although practically employed on alaboratory basis, does present practical problems when it is scaled upfor production of commercial quantities of metal powders. Moreover, theproducts produced thereby are still not at their optimum order. Analternative, less cumbersome, method of achieving very small, yetmagnetic, particles isdesirable.

SUMMARY OF THE INVENTION Therefore, it is an object of the invention toprovide a novel process for making very small magnetic partimizing anysintering of said particles.

Other objects of the invention will be obvious to those skilled in theart on reading this application.

The above objects have been accomplished by heat treating smallferromagnetic metallic powders at temperatures high enough to permit acrystalline or atomic rearrangement within the metal particle.

The most advantageous means for producing said powders is believed to beby reduction of a soluble metallic salt to the metal by a strongreducing agent, eg a "metal borohydride like sodium borohydride.However, formation of metal particles by decomposition of metalcarbonyls or by other precipitation reactions, may also be used toprovide particles for use in the invention.

The term ferromagnetic as used herein is meant to cover not only alphairon, cobalt, nickel, gadolinium and dysprosium, but is also meant tocover materials such as the Heusler-type alloys which are ferromagnetic,albeit the individual elemental components thereof are not. These lattercompositions include manganese alloys with copper and aluminum orindium, with arsenic and with antimony. The manganese atom therein isgenerally considered to contribute the ferromagnetic activity in thealloy environment.

Heat treatment temperatures in the range of from 250C to 650C haveproven effective with cobalt, iron, and mixtures thereof. Those skilledin the metallurgical arts will understand that the heat cycle will havevarious equivalents when higher temperatures are used for shorter timesor when lower temperatures are used for longer times.

In a particularly advantageous embodiment of the invention, the thermaltreatment is carried out while maintaining the metal particles inintimate contact with a refractory substance, say a high-melting saltlike sodium chloride. The presence of such a refractory material hasbeen found to sub-stantially inhibit excessive sintering of the metalparticles during heat treatment. The presence of the refractory materialis particularly important at higher temperatures at which the particlestend to undergo a favorable realignment more readily. At suchtemperatures, the tendency for particulate sintering is particularlyhigh.

The refractory substance should usually have some physical or chemicalproperty which makes it readily separable from the magnetic particles.Mere differential magnetism is not usually a convenient means althoughit can be utilized for magnetic separation. Differential specificgravity can be exploited in a centrifugal separation process. It isusually more advantageous to use a chemical parameter such as watersolubility to effect the separation of the refractory material from themetal. Thus water-soluble salts such as sodium chloride, etc, are mostadvantageously used.

ILLUSTRATIVE EMBODIMENT OF THE INVENTION In order to point out morefully the nature of the present invention, the specific embodiments ofthe present process and products produced thereby are set forth below:

Working Example 1 A quantity of 71.4 grams of CoCl .6I-I,O was dissolvedin 300 milliliters water. In a second vessel, 11.4 grams of sodiumborohydride were dissolved in 300 milliliters of water.

The borohydride solution was charged into the cobalt salt solution andagitated with a magnetic stirrer. This resulted in the reduction of thecobalt to a particulate 85 oersteds 0.35

Saturation Magnetic Moment Coercivity Squareness The magneticmeasurements set forth above and elsewhere in this specification weremade on a vibrating string magnetometer. The samples were air-stablebecause they had been exposed to air before measurement and therefor hadan oxide protective layer thereon. Generally, two hysteresis loops weretraced for each sample; one at about 1 kilooersted peak-field and one atabout 8 kilo-oersteds. This methodis quantitative and requires only asmall sample, e.g. several milligrams of the magnetic powder beingmeasured.

Squareness is defined as the remanent magnetization divided by thesaturation magnetic moment. Thus, remanence can be calculated by simplymultiplying squareness by saturation magnetization. A sample of 500milligrams of the resulting particles were heat treated at 300C under ahydrogen atmosphere for 30 minutes, cooled to room temperature whilestill under the hydrogen atmosphere, and purged with argon gas.Thereupon the magnetic properties were measured to be as follows:v

48 e m u/gram 356 oersteds 'Working Example 2 A quantity of five gramsof the cobalt powder, taken from the product of Working Example 1 beforethe heat treatment thereof, was mixed with 80 grams of very fine sodiumchloride powder. This salt powder had all passed through a 425 meshscreen.

The particulate mixture was comminuted in a quartcapacity ball-mill jarwith ceramic balls for 48 hours. Thereupon, eight grams of the powderymixture was subjected to the same heat treatment as the materialdiscussed in Example 1, i.e. 300C for 30 minutes.

After the heat treatment the powder mix was washed four times with 100milliliters of water and three times with 100 milliliters oftetrahydrofuran. The material was air dried, vacuum dried and itselectromagnetic properties were measured to be as follows:

Saturation Magnetic Moment 67 e m u/gram Coercivity 560 oerstedsSquareness 0.38

The presence of the particulate sodium chloride during the heat-treatingstep resulted in about a 38 per cent increase in the saturation magneticmovement value and nearly a sixty per cent increase in the coerciveforce value over those values obtained when the salt was not presentduring the heat treatment, i.e. in the heat treatment of Example 1.

Working Example 3 Effect of increasing Temperature In addition to theheat treatment at 300C, described in each of Examples 1 and 2, thematerials from each Example were heat-treated for thirty minutes at aseries of higher temperatures with the following results:

Saturation Magnetic Moment Temperature Example 1 Example 2 No Salt SaltPresent From the above results, it seems clear that further increases intemperature will not enable obtaining higher saturation magnetic momentvalues during a cobalt powder treatment period as long as 30 minutes.

Coercive Force The optimum coercivity appears to bereached, during a30-minute treatment of cobalt at a temperature below 450C.

Squareness Temperature Example 1 Example 2 No Salt Salt Present 300C0.41 0.38 350C 0.42 0.35 400C 0.38 0.35 450C 0.18 033 550C 0.14 033 650C0.08 0.28

temperatures are (1) generally more favorable than could be achieved atany temperature for the salt-free powders and (2) more resistant tothermal decay than the properties of the salt-free powders.

Working Example 4 A quantity of 71.4 grams of CoCl .6l-l,O was dissolvedin 300 milliliters of water with 320 grams of sodium chloride which hadpassed a 425-mesh screen. This first solution was prepared in a WaringBlendor.

A second solution, containing 1 1.4 grams of sodium borohydride inmilliliters of water, was prepared in a separatory funnel and then addeddropwise to the Waring Blendor at the slowest agitation speed. Theaddition was slow enough to keep the exothermic reaction between the twosolutions from increasing the temperature of the reaction medium muchabove 35C. Cobalt precipitated.

The precipitate, which included a large part of the original salt, wasfiltered, washed with 400 milliliters of acetone twice, redispersed in400 milliliters of acetone, and air dried.

Forty grams of the dried material was enclosed in a glass container andheated to 350C in a hydrogen atmosphere, maintained at 350C for an hour,cooled while still under hydrogen, purged for 2 minutes with SaturationMagnetic Moment 91 e m u/gram Coercivity 500 oersteds Squareness 0.32

Working Example 5 Seventy-five milliliters of a 0.7 molar cobaltchloride and 0.3 molar FeCl solution is charged into a small stainlesssteel mixer which is maintained in a strong magnetic field of about1,500 oersteds. A quantity of 75 milliliters of a one molar, aqueous,sodium borohydrate solution is slowly added to the solution of metalchlorides. The resultant reaction results in the precipitation ofmetallic particles which werewashed several times with water, washedwith tetrahydrofuran, separated from the liquid with a magnet after eachwash, and air dried.

The magnetic properties of the resulting cobalt-iron mixture weremeasured to be as follows:

Saturation Magnetic Moment 40 e m u/gram Coercivity 312 oerstedsSquareness 0.42

Heat treatment data will be given under Example 8. Working Example 6When the Example 5 was substantially repeated, but with 75 millilitersof a chloride solution 0.9 molar in cobalt chloride and 0.1 molar inFeCl the following magnetic properties were measured on the resultingproduct:

Saturation Magnetic Moment 33 Coercivity 88 Squareness 0.35

Heat treatment data will be given under Example 8. Working Example 7 vThe, powder product described in Example 5 is mixed with 16 times itsweight of sodium chloride powder which had passed a 425 mesh screen.Eighty-five grams of the resulting mixture was dispersed in 300milliliters of ethyl alcohol and ball milled for 24 hours using stoneballs and a l-quart capacity ceramic ball mill, then recovered anddried.

Heat treatment data will be given under Example 8. Working Example 8This example describes the heat treatment of the powders which wereproduced by the procedure described in Examples 5, 6 and 7. I

Small samples of each of these materials were heat treated at varioustemperatures under an argon atmosphere. Each sample held at thetreatment temperature for 80 minutes under an hydrogen atmosphere afterbeing brought up to temperature under an argon atmosphere. Then thesamples were cooled under hydrogen, and purged with argon. Each cooledsample was washed four times, with water, washed four times withtetrahydrofuran, air dried and dried further under a vacuum.

1n the following table the values of magnetic properties of thenon-heated materials are placed in parentheses.

Treatment Temperature of 250C Saturation Product Magnetic CoercivitySquareness Source Moment Example 5 48 (40) (312) 0.43 (0.42) Example 658 (33) 200 (88) 0.44 (0.35) Example 7 28 457 0.42

Treatment Temperature of 350C Saturation Product Magnetic CoercivitySquareness Source Moment Example 5 81 (40) 850 (312) 0.41 (0.42) Example6 7.4 (33) 250 (88) 0.41 (0.35) Example 7 76 712 0.37

It is clear, therefore, that the process of the invention provides ameans to improve the magnetic properties of particles produced byreduction of dissolved metal salts within a strong magnetic field.

It is of course to be understood that the foregoing examplesare intendedto be illustrative and that numerous changes can be made in the reactantproportions, and conditions set forth therein without departing from thespirit of the invention as defined in the appended claims.

What is claimed is:

1. In a process for making a magnetic metallic powder of the typecomprising a major portion of cobalt metal wherein said processcomprises the step of forming metal particles by reacting a reducingagent with a metallic salt in solution and a subsequent thermaltreatment of dried metallic powder formed of said particles, theimprovement comprising the steps of A. intimately mixing a quantity ofinorganic refractory salt powder material with said dried metalparticles, said quantity being an amount effective to separate saidmetal particles from one another to markedly reduce the physicalinteraction of said metal particles during heating then B. heat treatingthe resulting mixture at a temperature above 250 C. to enhance themagnetic properties thereof, and then C. separating said metal particlesfrom said refractory powder thereby providing a mass of magneticmetallic particles of enhanced magnetic properties.

2. A process as defined in claim 1 wherein said salt dust iswater-soluble salt.

3. A process as defined in claim 1 wherein ing is carried out for lessthan one hour at a temperature of about 450C or below.

4. A process as defined in claim 1 wherein said heat treating is carriedout at a temperature of about 350C or below.

5. A process as defined in claim 1 wherein said heating is carried outuntil the coercive force exceeds 500 oersteds. I

6. A process as defined in claim 1 wherein said heating is carried outuntil the coercive force exceeds 500 oersteds.

7. A fine magnetic powder of cobalt and formed according to the processdefined by claim 1.

said heat-

2. A process as defined in claim 1 wherein said salt dust iswater-soluble salt.
 3. A process as defined in claim 1 wherein saidheating is carried out for less than one hour at a temperature of about450*C or below.
 4. A process as defined in claim 1 wherein said heattreating is carried out at a temperature of about 350*C or below.
 5. Aprocess as defined in claim 1 wherein said heating is carried out untilthe coercive force exceeds 500 oersteds.
 6. A process as defined inclaim 1 wherein said heating is carried out until the coercive forceexceeds 500 oersteds.
 7. A fine magnetic powder of cobalt and formedaccording to the process defined by claim 1.